1. Field of the Invention
This invention relates to room temperature storage-stable, heat-curable mixtures containing polyisocyanates and active hydrogen compounds.
2. Description of the Prior Art
The preparation of optionally fiber-reinforced, cellular or noncellular molded parts or semi-finished products for such parts from compounds having reactive hydrogen atoms and polyisocyanates is described in numerous patents and patent applications.
In British patent GB No. 1,209,243, polyurethane foam molded parts are prepared by mixing organic polyisocyanates, compounds having reactive hydrogen atoms, blowing agents and catalysts at room temperature, the expandable reaction mixture is placed in a temperature-controlled, closable mold in large enough amounts to assure that the mold is completely filled out with the foam and such that the foam can expand and cure under compression.
In order to improve the mechanical properties of such molded foam parts, fibers or fiber mats as well as easily surrounded reinforcing pieces are placed in the mold and are embedded in the molded polyurethane part as it forms during the subsequent addition polymerization reaction and during foam expansion.
The disadvantage of this method is that the starting components must be added separately to the mixing unit and must be metered individually. Another disadvantage is that the addition polymerization begins directly upon mixing of the starting components. In particular, it is difficult to incorporate fiber materials which have a reinforcing effect, for example glass fiber mats, which must be completely saturated with the optionally expandable reaction mixture before the reaction begins.
These problems also could not be eliminated by using a two-component process, in which the compounds having reactive hydrogen atoms, catalysts, and optionally blowing agents, auxiliaries, and additives are usually combined to form the A component and the isocyanate group-containing compounds usually comprise the B component. The advantage of this procedure is that only two components need to to stored, metered in, and mixed. However, because of the rapidly climbing viscosity of the reaction mixture, long-fiber and planar reinforcing materials are not impregnated uniformly. This causes holes and voids, which sharply reduces the strength of the finished product.
Single-component polyurethane systems can be prepared by using capped polyisocyanates. These adducts are relatively thermal stable. Upon heating, the capping agent is cleaved off and the compound undergoes addition with reactive hydrogen atoms producing a thermally stable final product. This process is described, for example, in the Kunststoff-Handbuch, vol. VII, Polyurethane by R. Vieweg and A. Hochtlen, Munich: Carl Hanser Verlag, 1966, p. 11 ff., and it is used for the preparation of baked enamel as described in German published applications No. DE-A-26 12 638 (U.S. Pat. No. 4,068,086), No. DE-A-26 39 491 and No. DE-A-26 40 295.
The importance of such single-component systems is primarily determined by the type of capping agent which is used. With paint systems this agent evaporates and must be separated from the exhaust air, or it remains in dissolved form in the resulting high molecular weight addition polymerization product and affects its mechanical properties.
Various methods are described for overcoming these disadvantages. In the specifications of U.S. Pat. No. 3,475,200, storage-stable compositions are described which cure by heat to form polyurethane planar structures, coverings, or coatings. Said compositions are prepared from polyols, uretidione diisocyanates which melt at temperatures in excess of 100.degree. C., and chain extenders which melt at temperatures in excess of 80.degree. C., whereby at least 80 percent of the total amount of particles of the uretidione diisocyanate and the chain extender must be less than 30 .mu.m in diameter.
Uretidione group- and end-capped isocyanate group-containing organic compounds are prepared as described in GB 1,488,631 by reacting a uretidione group-containing polyisocyanate with an excess amount of a chain extender having a molecular weight of from 18 to 300 and with from 2 to 3 reactable groups relative to the isocyanates.
The capping agent, which is liberated during crosslinking, also remains in the addition polymerization product in this case.
U.S. Pat. No. 3,248,370 describes thermoplastic polyurethanes prepared from difunctional hydroxyl compounds, uretidione diisocyanate, and a second diisocyanate as well as glycol or water as the chain extender at temperatures under 100.degree. C., whereby the starting components are reacted in such amounts that nearly all the free isocyanate groups are consumed.
The uretidione group-containing process products described in the cited publications are, according to the examples, generally based on dimeric toluene diisocyanates and already in the non-cured condition they represent high molecular-weight, thermoplastic precursors having reactive groups.
Not until 140.degree. C. is exceeded do the precursors react--as they soften or melt--to form high molecular-weight or crosslinked polymerization products. This is because the uretidione group is sufficiently reactive at these temperatures and the reactants, which are initially fixed in the polyurethane matrix, can come in contact due to diffusion processes and can therefore undergo addition polymerization.
On the other hand, if the starting components cited in the above-referenced publications, for example, dimeric toluene diisocyanate, polyester polyols, and polyether polyols are mixed with or without low molecular weight chain extenders and without premodification, high molecular weight products are obtained which can only be reacted by using elaborate processing techniques involving the crosslinking of the uretidione groups to form crosslinked polyurethanes. In some cases, the use of such techniques is completely impossible. Because of the relatively low solubility and the relatively high melting point (approximately 150.degree. C.), reaction mixtures having a medium-length or even relatively long processing time can be prepared from dimeric toluenediisocyanates and conventional polyols and chain extenders at an index of 100. However, after the necessary catalysts have been added, these mixtures are no longer storage-stable at room temperature.
Canadian Patent No. CA 1,147,123 described a process for the preparation of fiber-reinforced molded parts in which the polyurethane compositions based on high melting point polyisocyanates, for example dimeric toluene diisocyanates, are reacted at specific quantitative ratios of the starting components. The disadvantage here is that the polyurethane compositions are only storage-stable for from several hours to a maximum of a few days.
Solvent-free molding compositions of a hydroxyl group-containing prepolymer having from 0.5 to 7 weight percent free hydroxyl groups and from 5 to 20 weight percent urethane groups, polyisocyanates having a melting point in excess of 100.degree. C., and organic or inorganic fillers in specified quantitative ratios are described in U.S. Pat. No. 4,251,428. The disadvantage of this method is that only special hydroxyl group-containing prepolymers are suitable as the compounds having reactive hydrogen atoms for preparing the molding compositions and the products are only moderately storage-stable.
The U.S. Pat. No. 4,251,427 also relates to coating compositions which contain a polyisocyanate having a melting point in excess of 100.degree. C., a hydroxyl group-containing prepolymer prepared from a polyisocyanate, a partially branched polyether polyol, a glycol having a molecular weight of from 62 to 250, and a compound having at least two amino groups, a molecular sieve of the sodium aluminum silicate type, and an activator. If the coating composition is not prepared from pre-extended polyols, the composition is not storage-stable when dimeric toluenediisocyanates are used in the presence of catalysts.
Long-term storage-stable, heterogeneous single-component systems of polyols and diphenylmethane uretidione diisocyanates are the subject of European published application EP-A-71898.
In addition to specific undesired properties, for example high viscosities, the high temperature required to cleave the capping agent and to achieve crosslinking, the described molding and coating compositions also have the disadvantage that only very specific polyisocyanates with very specific polyols can be combined to prepare said compositions.
Storage-stable, heat-curable compositions suitable for use as coating and adhesive bonding agents as well as sealants, in which the isocyanate is present in form of discrete particles in the polyol and whose particle surfaces are deactivated, is also described in European published application No. EP-A-62780. As befits this range of applications, however, this process only results in flexible, adhesive products, which are not suitable for the preparation of molded parts due to their inadequate mechanical properties after curing.